Wear-resistant titanium and titanium alloys and method for producing same



Oct. 7, 1969 R. A. wooo WEAR-RESISTANT TIT ANIUM AND TITANIUM ALLOYS AND METHOD FOR PRODUCING SAME Flled July 29, 1966 3 Sheets-Sheet 5 3 2?: $28 222250 22 372m; 528 WEE; 22225 mi $55G 2921.551 mmmzoEE 5 2025328 cow -oom

IN! 'E.\' TOR. RICHARE A WOOE ovow ol ux A 4, 3M4 a/nzm gm r AT TO RN EYS United States Patent U.S. Cl. 148-20.?) 14 Claims ABSTRACT OF THE DISCLOSURE Producing titanium or titanium alloy having a hard surface region of high adherence by plating the surface with chromium to a maximum thickness of 0.5 mil, diffusion annealing the plated article to diffuse substantially completely the chromium into the substrate, and heating the composite in a nitrogen atmosphere until a nitgrogen-stabilized alpha titanium-chromium alloy surface layer is formed.

An article comprising titanium or titanium alloy ba e metal having improved hardness comprising a body of said base metal and a diffused zone adjacent to the surface of said base metal including a nitrogen-stabilized alpha layer of titanium-chromium alloy is also disclosed.

The present invention is concerned with titanium and titanium alloy articles having hard, Wear-resistant surfaces. The invention is also concerned :with methods for producing such articles. The present application is closely related to a copending application of the same assignee entitled Wear-Resistant Titanium and Titanium Alloys and Method for Producing Same, filed July 29, 1966, in the name of H. R. Ogden, and assigned U.S. Ser. No. 568,903.

It is well recognized that titanium and titanium alloys possess properties of strength and light weight which make them highly desirable for a variety of uses. For example, such metals have wide application in the space and aircraft industries and more recently are finding utility in components for high speed business machines.

Despite the basic advantages of titanium and its alloys for many purposes, their resistance to wear is notoriously inadequate for applications involving friction contact. In an attempt to overcome this problem, the metal and its alloys have been provided with various coatings of other metals to alleviate the condition of wear and galling.

However, prior attempts to coat titanium and titanium alloys have generally been unsatisfactory for a number of reasons. In some cases, the resulting coating or surface layer did not provide hardness on the order sought. In other instances, the coating was not sufiiciently adherent to the base metal, especially where the metal was subsequently subjected to solution heat treating and aging to improve yield strength or other properties.

In the above-identified, related copending application, titanium and titanium alloy articles having improved wear resistance and a method for preparing such articles are disclosed. The present invention is directed to an improvement over the invention described in the copending application by which increased surface hardness and depth of the hard, surfacing material is achieved.

Accordingly, an object of the present invention it to provide titanium and titanium alloy articles having high surface hardness and improved wear resistance. A collateral object is to provide titanium and titanium alloy articles which have the desired properties and which remain substantially unimpaired by solution heat treatment and aging of the base metal.

-A furtherobject of the present invention is to provide a method for producing titanium and titanium alloy articles having high surface hardness and improved resistance to wear, even where the articles are subjected to subsequent solution heat treatment and aging.

An additional object of the invention is to provide titanium and titanium alloy article's having a surface zone which has improved wear resistance and remains highly adherent to the base metal despite subsequent impact, wear and thermal treatments.

The foregoing objects and many other highly desirable advantages are achieved in accordance with the present invention which comprises, in general, plating a titanium or titanium alloy base metal with a layer of chromium, diffusion annealing the chromium plated base metal to form a diffused titanium-chromium alloy zone on the surface of the base metal and nitriding the article. It is preferable that the initial plated chromium layer be kept sufficiently thin so that during the diffusion annealing step, the chromium is substantially completely diffused into the workpiece.

As a final step, the chromium-diffused, nitrided article can then be solution treated and aged or otherwise thermally exposed for the development of base metal properties.

As a result of this treatment the titanium or titanium alloy base metal is provided with a surface zone enriched with diffused chromium and nitrogen. The surface zone is hard and strongly adherent to the base metal. Subsequent heat treatments, as in solution treating and aging, do not adversely affect the properties of the article. The overall depth of the surface zone usually is on the order of about 5 mils, but may extend up to about 15 mils.

The surface zone formed during the process is composed of a series of layers. Considering the layers in order, proceeding inwardly from the exposed surface of the article, they are as follows:

(a) A nitrogen-stabilized alpha layer of titanium chromium alloy, possibly containing some TiN, i.e., titanium-nitrogen compound;

(b) A chromium-stabilized stable beta layer which may contain some nitrogen-stabilized alpha grains; (c) A chromium-stabilized transformed beta layer which may contain a few nitrogen-stabilized alpha grains, and (d) A chromium enriched alpha-beta layer.

Layer (d) overlies the titanium or titanium alloy base metal.

The invention is described more fully below with reference to the accompanying drawings wherein:

FIGURES 1 and 2 are photomicrographs and associated explanatory legends and hardness penetration data for articles in accordance with the invention, and

FIGURE 3 is a graph comparing the surface hardness of the titanium alloy article illustrated in FIGURE 1 with the titanium alloy article prepared by the process of the above-noted copending application U.S. Ser. No. 568,903.

The present invention is applicable to titanium metal and titanium-base alloys. A titanium-base alloy is generally considered to be one containing at least about 50% by weight titanium. For example, the invention may be applied to a Ti-6Al-4V alloy.

The titanium and titanium alloys of interest have potental value in the fabrication of components for high speed business machines. For example, Ti-6Al-4V alloys are light-weight and can be heat treated to high strength levels on the order of 160170K s.i. (K s.i.: 1000 p.s.i.). However, the wear resistance of titanium and its alloys is generally unsatisfactory for friction contact applications. In order to improve the surface hardness in accordance with the present invention, samples of a Ti-6AI-4V alloy were treated.

MATERIAL The samples were in the form of rods having a diameter of approximately 0.25 inch. Ti-6Al-4V alloy samples of the following composition were employed.

PRELIMINARY PREPARATION (1) The titanium alloy rods were chemically polished in a H-F-HNO solution to remove disturbed metal from the ground surface. A very slight reduction in the diameter of the samples results from the chemical polishing.

(2) The rods were then alkaline degreased and rinsed in commercial alkaline cleaner and tap water.

(3) The rods were repolished in the following solution at 85 F. for twenty minutes:

NH FHF ..g./l 100 H SiF (31%) ml./l 200 HNO (70%) ml /1 400 H O (distilled) Bal.

After polishing, the samples were rinsed in tap water.

(4) The rods were immersed in the activation solution described below, at 120 F. with an anode current density of 20 amps/square foot and a titanium cathode. Initially an activation period of 40 minutes was employed, but this was found to produce a rough surface. Later a shorter period of about 2 minutes was adopted and found to be more satisfactory in providing a smoother surface.

ACTIVATION SOLUTION Ml./l. Glacial acetic acid 875 Hydrofluoric acid (48%) 125 After activating, the samples were rinsed in tap water.

CHROMIUM PLATING The titanium alloy rods were electroplated using the chromium plating solution and conditions set forth below:

Chromic acid (CrO 250 g./l. Sulfuric acid (H 80 2.5 g./l. Distilled water Balance.

4 Temperature 130 F. Current density 300 amps/sq. ft. Anodes 93% Pb7% Sn. Plating rate 1 mil per min.

Electroplating is the chromium deposition method of choice, but other techniques, producing a firmly adherent plate of the desired thickness, are acceptable. Thus, vapor plating or other comparable procedure may be followed.

The exact conditions of plating and diffusion annealing may be varied somewhat and this will have an effect on the nature of the layers formed. Likewise, variations in the titanium base metal composition will result in different, but comparable results. In general, the titanium base metal is plated with a layer of chromium having a thickness such that it may be substantially completely diffused into the base metal by thermal treatment of practical duration and intensity. Thus, the chromium plating should have a maximum thickness of about 0.5 mil and preferably 0.3 mil or less.

DIFFUSION ANNEALING The thermal diffusion should be conducted for sufficient time to diffuse the chromium layer substantially completely into the base metal. A thermal diffusion conducted fora period of up to about 24 hours at a maximum temperature of about 1900 F. is generally satisfactory. The annealing temperature ordinarily will not be less than about 1600 F.

The articles may be diffusion annealed by placing them in sealed capsules containing a substantially inert atmosphere and heating for the desired period. For example, the article may be placed in an argon-filled Vycor capsule. Annealing temperatures on the order of from about 1600 to 1900 F. are preferred.

NITRIDING Nitriding of the article generally may be accomplished by heating the articles in a nitrogen atmosphere at a temperature above about 1500 F. for a period of about hour or more. For example, the articles may be placed in a nitrogen-filled Vycor capsule and heated for 1 hour at 1700 F.

In general, nitriding may be conducted by known techniques, following the diffusion treatment.

SOLUTION TREATING AND AGING For example, the base metal may be solution treated by heating at temperature about 50 F. below the beta transus, holding at this temperature for about /2 to 2 hours and water quenching. This may then be followed by aging which ordinarily is conducted for from 4 to 20 hrs. at a temperature of from 900 to 1000 F. The solution heat treating and aging treatments which may be employed in this invention are in accordance with standard practice for titanium and titanium alloys.

It will be apparent to those skilled in the art that the diffusion annealing, nitriding, solution treating and aging steps of the invention involve overlapping temperatures. Therefore, two or more of these operations may be conducted concurrently, as illustrated in connection with the second example described below. In that case the nitriding and solution treating steps are combined.

Example 1 A first sample, prepared as described above, using an activation period of 40 minutes and having a chromium plate of 0.05 mil thickness was treated according to. the following schedule:

(a) Diffusion annealed for 16 hrs. at 1700 F. and air cooled,

(b) Nitrided for 4 hrs. at 1750 F. and air cooled,

(c) Solution heat treated for V2 hr. at 1750 F. and Water quenched, and

(d) Aged for 6 hrs. at 1000 F. and air cooled.

Hardness data for four traverses of the rod appear in the following table, Table I. Hardness on the order of 1200 Khn. was observed in the near-surface region.

TABLE I.HARDNESS PENETRATION DATA [Khn. vs. Distance From Surface] Knoop Hardness 10 g. load 100 g. load Traverse N0 (1) (2) (3) i (4) Example 2 A second sample, prepared as described above, using a 40 minute activation and having a chromium plate of 00.5 mil thickness, was treated according to the following schedule:

(a) Difiusion annealed for 16 hrs. at 1700 F. and air cooled,

(b) Nitrided and solution treated in one treatment of /2 hr. at 1750' F. and water quenched, and

(c) Aged for 6 hrs. at 1000" F. and air cooled.

Hardness data for three traverses of the treated rod appear in the following table, Table II. Hardness on the order of 1300 Khn. was observed in the near-surface region.

TABLE IL-HARDNESS PENETRATION DATA [Khn. vs. Distance From Surface] Knoop Hardness 10 g. load 100 g. load Traverse No (1) (2) (3) Example 3 A third sample prepared as described above, using an activation period of 2 minutes and having a chromium 75 plate of 0.05 mil thickness, was treated according to the following schedule:

(a) Diffusion annealed for 15 hrs. at 1700" F. and air cooled,

(b) Nitrided for 1 hr. at 1700 F. and air cooled,

(c) Solution heat treated for /2 hr. at 1700= F. and water quenched, and

(d) Aged for 6 hrs. at 1000 F. and air cooled.

FIGURE 1 of the drawing is a photomicrograph of a cross-section at the surface of the third sample prepared as set forth above.

The black material at the top of the photomicrograph is mounting material in which the rod was placed prior to taking the cross-section.

The legends to the right of the photomicrograph identify the layers formed in the sample at the depths indicated as a result of the process.

The surface layer of the Ti-6Al-4V alloy article is a difiused nitrogen-stabilized alpha layer, possibly containing some titanium-nitrogen compound. This layer extends only a fraction of a mil in depth, as can be seen by reference to the Scale of Photo, to the right of the legends. This scale indicates the approximate depth in mils of each of the layers and of the overall surface zone.

The second layer is a diffused chromium-stabilized stable beta layer with some nitrogen-stabilized alpha grains. This layer is about 1 mil in depth.

The third layer is a diffused chromium stabilized transformed beta layer with a few nitrogen-stabilized alpha grains. This layer is about 2 mils in depth.

The fourth layer is a chromium enriched alpha-beta layer somewhat less than two mils in depth and is firmly bonded to the base metal. In this sample, it is in essence a modified form of the base metal.

The overall surface zone, comprising the four layers described above is about 5 mils in depth.

Hardness penetration data. for the sample is also set forth in FIGURE 1 and is keyed to the photomicrograph by the Scale. From this data it is seen that very high hardness is obtained at the surface of the sample, decreasing through the intermediate layers to the hardness of the base metal.

The hardness penetration curve for this chromiumnitride coated sample is presented in the graph of FIG- URE 3 of the drawing. This curve is compared with the hardness penetration curve of a diffused-chromium treated sample which was prepared in accordance with the previously mentioned related application, Ser. No. 568,903 of Horace R. Ogden. The process of the related application does not include nitriding.

The curves demonstrate that while the chromium dif fusion treatment of the related application Ser. No. 568,- 903 provides improved surface hardness, the chromiumnitride process of the invention provides a much greater improvement in hardness, especially in the region very close to the surface.

The sample prepared by this invention showed a surface hardness in both traverses of well over 900 Khn. By comparison, the maximum hardness of the sample prepared by the chromium-ditfusion of application Ser. No. 568,903 is on the order of 700 Khn.

Example 4 Another sample was prepared as was the sample in Example 2, but the aging step was omitted. One hardness traverse was taken.

FIGURE 2 of the drawing includes a photomicrograph of a crosssection at the surface of this sample. Legends identifying the surface layers, hardness penetration data and a scale relating the data to the photomicrograph are also presented in the same manner as in FIGURE 1.

Again, it will be seen that very high surface hardness, on the order of 1000 Khn., is found in the resulting article. Surface layers of the same character as are shown in the photomicrograph of FIGURE 1 are also present in this sample. The data indicates high hardness in the surface layers formed by diffusion annealing of the chromium plate plus nitriding and subsequently solution heat treating.

It will be apparent to those skilled in the art that various modifications may be made in the process and article illustratively described above without departing from the scope of the invention as expressed in the following claims.

What is claimed is:

1. A method for producing an article of titanium or titanium alloy having a hard surface region of high adherence to the base metal comprising:

(a) plating the surface of the base metal with essentially only chromium to a maximum thickness of 0.5 mil, thereby forming an adherent layer on said base metal, I

(b) diffusion annealing the plated article resulting from step (a) to diffuse substantially completely said chromium into said base metal, and

(c) heating said articles in a nitrogen atmosphere until a nitrogen-stabilized alpha titanium-chromium alloy surface layer is formed.

2. The method of claim 1 further comprising solution treating said article.

3. The method of claim 2 further comprising aging said article.

4. The method of claim 1 wherein said base metal is a Ti-6Al-4V alloy.

5. The method of claim 1 wherein the diffusion annealing of step (b) is conducted for up to 24 hours at a temperature of from 1600 to 1900 F.

6. The method of claim 1 wherein said article is heated in said nitrogen atmosphere at a temperature above about 1500 F. for a period of at least about /2 hour.

7. A method for producing an article of titaniumor titanium base alloy having a hard surface region of high adherence to the base metal comprising:

(a) plating the surface of said base metal with a layer consisting essentially of chromium having a maximum thickness of 0.5 mil,

(b) diffusion annealing the plated article resulting from step (a) at a temperature in the range of from 1600 to 1900 F. to diffuse substantially completely said chromium layer into said base metal,

(c) heating said article in a nitrogen atmosphere at a temperature above about 1500 F. for a period of at least about /2 hour, and

(d) solution heat treating said article.

8. The method of claim 7 further comprising aging said article.

9. The method of claim 1 wherein said chromium plate has a maximum thickness of about 0.3 mil.

10. The process of claim 7 wherein said chromium plate has a maximum thickness of about 0.3 mil.

11. An article of titanium or titanium alloy base metal having improved hardness comprising: a body of said base metal, and a ditfused zone adjacent to the surface of said basemetal including a nitrogen-stabilized alpha layer of titanium-chromium alloy wherein said diffused zone comprises the following metal structures in order inwardly from said surface,

(1) a nitrogen-stabilized alpha layer of titanium-chromium alloy,

(2) a chromium-stabilized stable beta layer of titaniumchromium alloy,

(3) a chromium-stabilized transformed beta layer of titanium chromium alloy, and

(4) a chromium enriched alpha-beta layer of titaniumchromium alloy.

12. The article of claim 11 wherein said surface of said base metal has a hardness on the order of about 1000 Khn. and above measured under a 10 g. load.

13. The article of claim 11 wherein the difiused zone has a depth of up to about 15 mils.

14. The article of claim 11 wherein said base metal is a Ti-6Al-4V alloy.

References Cited UNITED STATES PATENTS 2,645,575 7/1953 Herres et al. 148-133 X 2,804,410 8/1957 Wyatt et al 148-133 X 2,819,958 1/1958 Abkowitz et al. 148-133 X 2,858,600 11/1958 Vigor 29-198 2,906,008 9/ 1959 Boegehold et al. 29-198 X 2,908,969 10/1959 Wagner 29-198 X 2,968,586 1/1961 Vordahl 148-133 3,268,306 8/1966 Jefferys 29-198 3,297,552 1/ 1967 Gisser et al. 29-198 CHARLES N. LOVELL, Primary Examiner US Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No- 3,471,34z Dated October 7, 1969 I Richard A. Wood It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the S ecification:

Column 2, line 4 change "it" to =is= Column 4, line 47 after "at" and before "temperature" insert =a= Column 5, line 60 in Table II, "566" should be =656= SIGNED AND SEALED MAR 3 "-1970 Edward H. Fletcher, Jr. I. J Attesting Officer mm 1- commissioner of Paton 

